This invention relates to vapor-heated rolls for drying or heating material passing thereover, and more particularly, it relates to vapor-heated rolls for handling variable drying loads along their length.
Rapidly rotating heated rolls are extensively used in continuous drying operations. Typical drying applications such as found in the paper industry require a uniform heat transfer or heat flux rate at each point along the cylindrical surface of the roll. In these processes, paper webs pass in a serpentine path over a series of rolls. In the case of rolls heated by condensation of vapor, such as steam, the rate of condensate generation at each point along the axis of the roll is uniform.
Other industrial drying processes treat webs or individual threadlines comprised of multiple filaments by laying the web or filaments onto one end of a drying roll and spirally advancing the product along the roll until it is removed from the opposite end of the roll to pass to the next treatment step in the process. In the manufacture of wet-spun yarns or those requiring aqueous washing or extracting before windup, a drying step is utilized wherein product enters upon a drying roll at a high moisture level and is progressively dried to the desired moisture level as it advances along the longitudinal axis of the roll. Conditions in the roll interior, where condensation is occurring, are different from those encountered in rolls used to dry paper. The heat transfer rate varies as the product is dried and therefore the rate of condensate generation varies. This requires a different method of managing condensate removal from the roll to compensate for the variable thickness of condensate which collects on the interior surface of the outside wall. The thicker the condensate layer, the lower is the heat transfer rate. Greatest heat transfer rate occurs close to the roll-end where yarn enters, and condensate build-up here severely reduces both the drying rate and production capacity. In the past, processes have depended on increased pressure difference between the supply and discharge sides of the roll heating chamber to convey condensate from the roll. This results in wasted steam since excess steam is required to convey condensate from the roll. This conveying steam is therefore unavailable to condense and provide energy for product drying. Prior methods of condensate removal did not minimize thickness of the film of condensate and therefore limited the rate at which product could be dried.